Emergency lighting system



May 16, 1967 D. RwGRAFHAM 3,320,473

EMERGENCY LIGHTING SYSTEM Filed Sept. 22, 1.)64

INVENTORI DENIS R. GRAFHAM,

BY %M 7 ,Q

HIS ATTORNEY.

United States Patent 3,320,473 EMERGENCY LIGHTING SYSTEM Denis R.Grafham, Auburn, N.Y., assiguor to General Electric Company, acorporation of New York Filed Sept. 22, 1964, Ser. No. 398,176 3 Claims.(Cl. 315-86) My invention relates to an emergency lighting system and,more particularly, to an emergency lighting system for producing lightupon the failure of a conventional power supply.

There are many places which must be constantly supplied with light toprevent the possibility of danger to persons utilizing the particularfacilities found in such a place. Places of this sort include restrictedareas where failure of a light source could promote panic, such as in anelevator cab; regions lighted to prevent the possibility of personalattack, such as corridors and entrance ways; and those areas lighted toprevent the possibility of accident, such as in narrow, steep or windingstairways.

Failure of the light source in any of these locations could greatlyincrease the chances of danger for any individual in such a location.Therefore, when the conventional power supply for the lighting system insuch a region fails, it is desirable to have some means for producingemergency light. This may be achieved by substitution of a stand-bypower supply, or by providing a completely separate emergency lightingsystem which detects a failure of the conventional power supply andutilizes this detected information to cause energization of theemergency light. This latter approach has the advantage that thelighting circuit may be engineered for the particular emergency supplyinvolved, rather than having the emergency supply shifted into a circuitdesigned for the conventional power supply. It is this separate type oflighting system that is the subject of my invention Most prior artsystems have depended upon the use of relays to switch the emergencypower supply into a circuit to energize and to provide emergency light.These prior art systems have the disadvantages that accompany the use ofmoving mechanical parts for switching control, such as sensitivity tovibrations, humidity, vapor and climatic conditions. By providing astatic switching circuit for controlling energization of the emergencylamp upon failure of the conventional power supply, in ac cordance withmy invention, the emergency lighting systern is unaffected by thesefactors.

Thus, it is an object of my invention to provide an improved staticswitching circuit for controlling the energization of an emergency lampin case of power failure.

Other objects and advantages of my invention will become apparent as thefollowing description proceeds and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming part of this specification.

For a better understanding of my invention, reference may be made to theaccompanying drawing, the single figure of which is a circuit diagram ofone embodiment of my invention.

Briefly stated, in one of form of my invention, a voltage-controlledswitching device such as a silicon controlled rectifier (SCR) is placedin series with an electrically energizable lamp which provides emergencylight in the event of failure of the main power supply. To achieve thedesired energization of the emergency lamp in case of power failure, acapacitor circuit is utilized to control the firing of the SCR. Duringnormal energization of the conventional light source by the main powersupply, the capacitor is charged in such a direction as to preventfiring of the SCR and, thereby, prevent energization of the 3,320,473Patented May 16, 1967 emergency lamp. A relatively slow discharge pathfor the capacitor is provided which does not affect normal operation.However, a failure of the power supply for a predetermined number ofcycles will permit discharge of the capacitor to the point that a firingvoltage will be applied to the gate electrode of the SCR. Application ofthe firing voltage to the SCR initiates conduction of the SCR andsubsequent energization of the emergency lamp by a battery. The batteryis charged on alternate half cycles of the main power supply through aseries diode and resistor halfwave charging circuit.

Referring now to the embodiment illustrated in the single figure of thedrawing, it is seen that the emergency lamp 1 is connected in serieswith the silicon controlled rectifier (SCR) 2. SCR 2 has an anode 3, acathode 4, and a gate 5.

Alternating current from a main power supply is introduced into thecircuit by means of transformer 6, which has a primary winding 7 and asecondary winding 8, having a first side 8a and a second side 8b. Thesecondary winding 8 and a battery 9 are connected in series with theemergency lamp 1 and the SCR 2.

In order to maintain SCR 2 in a non-conducting state during the timethat the primary 7 of transformer 6 is supplied with an alternatingcurrent, a capacitor 19 is connected to the gate electrode 5 of SCR 2.Capacitor 10 is charged through diode 11 and resistor 12. Diode 11 andresistor 12 are connected in series between the battery 9 and capacitor10. A discharge path for the capacitor 10 is provided by resistor 13which is relatively large compared to the combined resistance of diode11 and resistor 12.

To provide a path for trickle charging the battery 9 on alternate halfcycles of the alternating current appearing across secondary winding d,diode 14 and resistor 15 are connected in series between winding 8 andbattery 9. The series connection of diode 14 and resistor 15 is aconventional half-wave charging circuit, to maintain the battery at itsrated capabilities.

A path for the firing voltage to SCR 2 is provided by the diode 16. Toproduce the voltage necessary to fire SCR 2, resistor 17 is connectedfrom the gate 5 of SCR 2 to the other side of battery 9, so that aseries firing circuit is formed by resistor 13, diode 16 and resistor17.

The operation of the circuit will now be explained in detail. Duringnormal operation of the alternating current supply, the primary winding7 of transformer 6 is supplied with an alternating current signal. Thissignal is transformed and appears across the secondary winding 8 of thetransformer 6. During the half cycle that the side 8a of secondarywinding 8 is negative, there is a flow of current to the capacitor suchthat it is charged, through the diode 11 and resistor 12, with thepolarity indicated in the single figure of the drawing.

With this charge on the capacitor the diode 16 is prevented fromconducting and SCR 2 is maintained in a blocking state. Also during thishalf cycle of the alternating current, the battery 9 is provided with atrickle charge through diode 14 and resistor 15.

During the half cycle that side 8a of secondary winding 8 is positive,capacitor 10 tends to discharge through resistor 13. However, theresistance of resistor 13 is made sufiiciently large that the timeconstant for the capacitor 10 and resistor 13 is much greater than thetime constant for the capacitor 10 and the combined resistances of diode11 and resistance 12. Thus, during this half cycle of the appliedalternating current the discharge of capacitor 10 is not sufiicient topermit firing of SCR 2 through the diode 16.

If there is a power failure so that there is no alternating currentvoltage across the secondary winding 8, then there will be a continuousdischarge of capacitor 10 through resistor 13. After the charge on thecapacitor 10 has decreased sufficiently SCR 2 will be fired by battery 9through resistor 13 and diode 16. Firing of SCRZ places it in aconductive state, thereby energizing the emergency lamp 1 through theseries circuit of battery 9, emergency lamp 1, SCR 2 and secondarywinding 8. Thus, failure of the alternating current supply causesenergization of the emergency lamp 1 to insure continued illumination ofthe affected area during the power failure.

After the power failure has creased and normal A.C. power is returned tothe circuit, the emergency lamp 1 is automatically extinguished.Automatic extinction is obtained by commutation, or turn-off, of SCR 2,which occurs upon return of normal power, since the peak line voltageexceeds the battery voltage and biases SCR 2 to the off condition. Thus,another important feature of this invention is the automatic turn-off ofthe light upon renewal of normal energization.

I do not desire my invention to be limited to the particularconstruction shown and described, and I intend to cover allmodifications and changes within the spirit and scope of my invention bythe appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An emergency lighting system comprising: a series circuit including asecondary winding, a battery, an electrically energizable lamp and asilicon controlled rectifier; a capacitor to control the firing of saidsilicon controlled rectifier; means for charging said capacitor from anA.C. power supply through said secondary winding to prevent firing ofsaid silicon controlled rectifier; means for 4- discharging saidcapacitor upon the failure of said power supply to permit firing of saidsilicon controlled rectifier. by said battery, thereby energizing saidlamp from said battery.

2. An emergency lighting system as recited in claim 1 wherein saidcharging means comprises the series combination of a diode and a firstresistor; and said discharging means comprises a second resistor inparallel with the series combination of said diode and said firstresistor, and second resitor having a reistance relatively largecompared to the combined resistance of said diode and said firstresistor, whereby the discharge through said second resistor issufliciently low to prevent the capacitor from normally dischargingbelow the value necessay to prevent firing of said silicon controlledrectifier.

3. An emergency lighting system as recited in claim 2 and furtherincluding: the series combination of a second diode and a third resistorconnected across said battery and said secondary winding, whereby saidbattery is charged from said power supply.

References Cited by the Examiner UNITED STATES PATENTS 2,889,490 6/1959Paradise 315-86 X 3,114,095 12/1963 Palmer 320-39 3,143,729 8/1964 Power340253 3,159,755 12/1964 Duncan 307-88.5

JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

1. AN EMERGENCY LIGHTING SYSTEM COMPRISING: A SERIES CIRCUIT INCLUDING ASECONDARY WINDING, A BATTERY, AN ELECTRICALLY ENERGIZABLE LAMP AND ASILICON CONTROLLED RECTIFIER; A CAPACITOR TO CONTROL THE FIRING OF SAIDSILICON CONTROLLED RECTIFIER; MEANS FOR CHARGING SAID CAPACITOR FROM ANA.C. POWER SUPPLY THROUGH SAID SECONDARY WINDING TO PREVENT FIRING OFSAID SILICON CONTROLLED RECTIFIER; MEANS FOR DISCHARGING SAID CAPACITORUPON THE FAILURE OF SAID POWER SUPPLY TO PERMIT FIRING OF SAID SILICONCONTROLLED RECTIFIER BY SAID BATTERY, THEREBY ENERGIZING SAID LAMP FROMSAID BATTERY.